Jahnsite-(MnMnFe), Mn2+Mn2+Fe2+2Fe3+2(PO4)4(OH)2·8H2O (IMA 2018-096), is a new phosphate belonging to the jahnsite subgroup of the jahnsite group of minerals. It was found in the dumps of the Malpensata pegmatite mine close to the village of Olgiasca, Colico municipality, Lecco province, Italy by one of the authors (F.V.). The mineral occurs on the surfaces of a hydrothermal cavity as prisms elongated along a, weakly flattened on {010}, and twinned by reflection on {001}, up to 130 μm in size. The new species occurs in close association with rockbridgeite as a late stage product of alteration of primary phosphate minerals including graftonite-(Mn). Its color is dark brown and it is partially transparent with a vitreous luster. The streak is pale greenish brown. Jahnsite-(MnMnFe) is brittle with a good cleavage on {001}. Its Mohs hardness is 4 and the calculated density is 2.654 g/cm3. The mineral is optically biaxial (–) with α = 1.673, β = 1.685, γ = 1.689, and 2V (calc.) = 60 °, and it is non-fluorescent under 254 nm (short wave) and 366 nm (long wave). The empirical formula is: (Mn0.40Ca0.25Na0.05)Σ0.70Mn(Fe2+1.39Mn0.43Mg0.14Zn0.03)Σ2.00(Fe3+1.99Al0.01)Σ2(PO4)4(OH)1.35·8H2O. Jahnsite-(MnMnFe) is monoclinic, with space group P2/a and unit-cell parameters a = 15.1559(6) Å, b = 7.1478(2) Å, c = 10.0209(4) Å, and β = 112.059(4)° with V = 1006.11(6) Å3 for Z = 2. The eight strongest measured lines in the XRPD pattern are [d in Å, (I/I0), (hkl)]: 2.590 (100) forumla, 9.221 (89) (001), 2.840 (82) (320), 4.932 (78) forumla, 4.651 (78) (002), 3.971 (71) (211), 3.504 (64) forumla, and 3.295 (46) forumla. Its crystal structure was refined and conforms with the general structure described for jahnsite-group minerals.

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